1. Field of the Invention
The present invention relates to a device for thermally fixing toner images in copying machines, printers, facsimile machines, or the like.
2. Description of the Prior Art
One conventional device for thermally fixing toner images is shown in FIG. 1 of the accompanying drawings. The conventional thermal toner image fixing device includes a heater 9, a temperature fuse 10, a heater control circuit 6, and an AC power supply 7. The heater 9 is surrounded by a thermal fixing roller 8 for fixing toner to a sheet of paper. The temperature of the thermal fixing roller 8 is detected by a surface temperature sensor 1 that is positioned near the thermal fixing roller 8. A controller 11, which is in the form of a microprocessor, determines whether the temperature as detected by the surface temperature sensor 1 is higher than a predetermined temperature or not, and sends a control signal to the heater control circuit 6.
If the temperature as detected by the surface temperature sensor 1 is lower than the predetermined temperature, then the controller 11 applies a control signal to the heater control circuit 6, which then energizes the heater 9 thereby heating the thermal fixing roller 8.
Generally, in order to fix toner sufficiently to a sheet of paper in the thermal toner image fixing device, it is necessary that the viscosity of a toner layer contacting the sheet as well as a surface toner layer remote from the sheet be lower than a certain viscosity level (normally in the range of from 104 to 105 poise), and the toner be impregnated in paper fibers of the sheet under the pressure of a pressure roller that is pressed at the same time when the toner is heated.
Heretofore, the energization of the heater is controlled based only on the comparison between the predetermined temperature and the temperature as detected by the surface temperature sensor 1. The predetermined temperature remains the same irrespective of various conditions that affect the fixing of the toner. Specifically, the toner is fixed at the same temperature even if the sheet has a different paper quality, the water content of the sheet varies due to a relative humidity around the copying machine, the printer, or the facsimile machine, and the temperature of the sheet varies depending on the ambient temperature of the copying machine, the printer, or the facsimile machine. When these conditions vary, however, the toner is fixed to different degrees.
For example, when the thermal fixing roller is heated to the same temperature, the toner fixation is poorer as the surface roughness of the sheet is greater. This is because the toner that has been softened below a certain viscosity level is not well impregnated in the paper fibers of the sheet since the wettability of the paper fibers with respect to the softened toner is lower as the surface roughness of the sheet is greater. If the surface roughness of the sheet is greater, then the paper fibers tends to be peeled off more easily when the fixed toner is subjected to shearing or peeling forces. When the shearing or peeling forces are applied, the fixed toner as well as the paper fibers held in contact therewith is peeled off.
The toner fixation is also poorer as the sheet has a higher water content. If the water content of the sheet is higher, then a larger proportion of the thermal energy that is applied to heat the toner and the sheet is consumed to evaporate the water in the sheet. Consequently, the toner and the sheet are not sufficiently heated. The temperature at the boundary between the toner and the sheet does not rise to a level necessary to fix the toner to the sheet, and the viscosity of the toner at the boundary does not decrease to a level necessary to fix the toner to the sheet.
If the ambient temperature is lower, then the temperature of the sheet is also lower, and the temperature at the boundary between the toner and the sheet does not rise to a level necessary to fix the toner to the sheet, resulting in poor toner fixation.